A "mil" is a common unit of measurement used to determine coating thickness. Special instruments called Mil or Coating Thickness Gauges are used to measure mil thickness.
1 mil is equal to 1 one-thousandth of an inch (25 microns).
The mil thickness of a coating not only effects the appearance of a coating but also the performance and protective qualities. A coating with a mil thickness that is too thin may have less than 100% coverage leaving the substrate vulnerable to corrosion. A coating that is too thick can loose flexibility and have lessened overall durability.
The mil thickness which works best for a coatings appearance can differ from what would provide the most protection.
Sometimes a choice must be made between protection vs. appearance based upon the conditions the part will be used in. For parts where aesthetics are the primary focus then a thinner coating may be desired for a smoother appearance. If protection is the primary goal then a heavier coat, or, multiple coats may be used, but poses a greater chance that orange peel will occur.